PyNodes: get active node tree from context in Blender 2.72
To get the parent of any node:

node.parent


To get the node, you can get it from the context active_node, if you can access the Nodes context, or, from the global context, get the scene and then the node from the correct node tree.

The function get_from_context you mention is a part of the NodeTree class. You would have to show your code for me to know what is the problem.